1. Field of the Invention
The present invention relates to an apparatus and method for producing molded polyester articles. More particularly, the invention concerns an apparatus and method for continuously producing molded polyester articles having low acetaldehyde content from a melt prepared by continuously reacting polyester precursors. The polyester is prepared and formed into useful shaped articles in a single, integrated, continuous melt-to-mold process without an intermediate solidification of the melt.
2. Description of the Prior Art
It is known in the art that polyesters are widely used in the manufacture of molded objects, such as food and beverage containers. These polymers are generally made by batch or continuous melt phase polycondensation reactions as are well know in the art. The polymers are then pelletized and used in various extrusion or molding operations. During the preparation and processing of polyesters in the melt phase, certain by-products are formed. One such by-product is acetaldehyde, and its presence in molded food containers, beverage bottles, water bottles, and the like is quite deleterious. Particularly in the case of taste sensitive beverages such as cola, beer, and water, it is highly desirable to produce preforms having less than about 10 ppm of acetaldehyde. Achieving this low level of acetaldehyde is difficult because acetaldehyde is continually formed as a by-product during the polymerization and subsequent melt processing of polyethylene terephthalate (PET) and similar polymers.
The prior art has made various attempts to reduce the acetaldehyde content of molded polyester articles. A three stage process has been universally used to provide polyester polymers suitable for uses in applications where it is important to minimize the presence of acetaldehyde. Such a process typically involves the preparation of a relatively low molecular weight precursor polymer by melt phase polymerization techniques that are well known in the art. The acetaldehyde content of such a precursor may range from about 30 ppm to in excess of 150 ppm, depending on the reaction conditions chosen. This precursor is then cooled, shaped into pellets, crystallized, and subjected to further solid-state polymerization at a low temperature. Typically, an inert gas is used to strip glycols, acetaldehyde, and other reaction by-products from the pellets so that at the end of the solid-state process, the acetaldehyde content has been reduced to below about 1 ppm or less. The product so prepared must still be heated and melted in a third step in order to be formed into a useful shape, such as a beverage bottle preform, and this process typically causes an increase in acetaldehyde content of from less than 1 ppm in the pellets, up to about 5 to about 10 ppm or more in the shaped article. This dramatic increase in acetaldehyde occurs despite the fact that the molding process takes typically less than one or two minutes to complete.
U.S. Pat. No. 5,597,891 describes an improved process for lowering the acetaldehyde content of molten PET to levels suitable for direct use in articles for food packaging by using an inert gas purge to contact the molten polyester. U.S. Pat. No. 4,734,243 describes an injection molding machine for plastics provided with a plasticizing device which consecutively plasticizes material and feeds a plurality of injection devices in regular sequence. German Patent DE19505680 describes a process for the production of bottle preforms from a melt wherein an inert gas is admitted into the continuous flow of the polyester melt and the acetaldehyde content is reduced to below 10 ppm. German Application DE19503053 describes the direct production of molded packagings that impart no taste from thermoplastic polyesters using conventional catalysts. Inert gas is dispersed into the polyester melt immediately after leaving the reactor, and vacuum degassed just before the molding machine. The inert gas is dispersed by means of stationary pipeline mixers and degassing is accomplished in an enlarged pipe segment or flash vessel under vacuum with an optional stirrer. Also, a polyamide type acetaldehyde reducing agent is added near the inert gas feed point. Japan Application Hei 5-315154 describes a method and apparatus for water injection, dispersion, bubbling, and degassing whereby volatiles present in the said polymer are vaporized and removed. U.S. Pat. No. 5,080,845 describes a method of removing low molecular weight substances from polymeric materials by mixing a supercritical extraction gas into the molten polymer under pressure in an extruder, and then transferring the mixture to a second extruder operated under reduced pressure where the gas, along with the extracted impurities, are vented and the polymeric material substantially free from impurities is discharged. U.S. Pat. No. 5,459,168 describes a process for treating an impure thermoplastic polymer using an extrusion apparatus to remove low molecular weigh compounds. The extrusion apparatus has a heated housing, in which there is a first low pressure zone, a high pressure zone where a separating agent is introduced into the molten polymer; an extraction zone where part of the separating agent and low molecular weight compounds are removed; a second low pressure zone, where the remainder of the low molecular weight compounds and separating agents are removed; a final high pressure zone; and an extruder head through which the purified polymer is recovered. U.S. Pat. No. 5,597,525 describes a method for injection molding polyethylene terephthalate by employing a vent type injection unit as means for omitting preliminary drying of the resin. The unit does not develop a poor screw biting, and even when the molding material is an undried, uncrystallized or crystallized PET, it can feed a certain amount of the material at all times to injection mold a molded form such as a preform to produce a thin-wall container. This patent discusses the need to keep the temperature of the molten polymer at a minimum in order to minimize formation of additional AA.
It has now been found that an apparatus and method may be employed whereby polyesters such as PET and similar polymers may be prepared continuously in a melt and transferred to this apparatus where it is purified of excess acetaldehyde and other by-products, and molded directly from the melt into useful shaped articles such as beverage bottle preforms, having a low acetaldehyde content. Not only does the apparatus of the present invention avoid the costly additional steps of conventional processing, such as cooling, chopping, drying, crystallizng, sold-state polymerization, and remelting, but the shaped articles produced additionally have low acetaldehyde content, better color, less loss of molecular weight due to polymer breakdown, and the freedom from defects know as "bubbles" and "unmelts" which are sometimes formed during the conventional molding process.